DL-2-amino-4-(hydroxymethylphosphinyl)butanoic acid (hereinafter, abbreviated as DL-AHPB) is a known compound having a herbicidal activity and used as an effective herbicide having a wide spectrum (Japanese Patent Application Laid-Open (JP-A) No. Sho52-139727). However, the herbicidal activity of DL-AHPB is about a half of that of L-AHPB, and it has been clear that a main substance of the herbicidal activity is L-AHPB (JP-A No. Sho55-000025 and JP-A No. Sho59-219297). Because of this, development of a process for producing L-AHPB selectively and effectively has been strongly desired.
Conventionally, as for the process for the production of L-AHPB, processes such as (a) a process with using a microorganism and an enzyme and (b) an asymmetric synthesis method are known.
As for examples of the process of (a), a process for producing L-AHPB from 4-(hydroxymethylphosphinyl)-2-oxobutanoic acid by using a transamination enzyme (Japanese Patent Application National Publication (Laid-Open) No. 2003-528572) and a process for producing L-AHPB from N-acetyl-DL-AHPB by using an enzymatic racemate resolution (Japanese Patent Application National Publication (Laid-Open) No. 2003-505031) are disclosed. However, there are problems in both of these processes being needed to carry out a reaction at a low substrate concentration, post treatment and purification steps are complicated, an expensive optically active amino acid has to be used at an equivalent mol or more in the transamination reaction, etc.
As for examples of the asymmetric synthesis of (b), a process for synthesizing L-AHPB by alkylation of (R)-3-isopropyl-2,5-dialkoxy-3,6-dihydropyrazine (JP-A No. Sho62-132891 and Tetrahedron Lett., 1255 (1987)) and a method of converting L-vinylglycine stereospecifically to L-AHPB (Tetrahedron, 8263 (1992)) are disclosed. However, it is necessary to use an expensive optically active amino acid such as D-valine and L-vinylglycine as a starting raw material, and there is a problem in the point of providing a raw material with low cost and in a large amount. Furthermore, an example of the asymmetric synthesis including a process for producing L-AHPB by an asymmetric hydrogenation reaction of 2-acetylamino-4-(hydroxymethylphosphinyl)-2-butenoic acid (JP-A No. Sho62-226993 and J. Org. Chem., 56, 1783 (1991)) is disclosed. In this process, the asymmetric hydrogenation reaction is performed using a rhodium catalyst having an optically active diphenylphosphine compound as a ligand. However, the rhodium catalyst is very expensive and catalytic efficiency is not high.
On the other hand, the asymmetric hydrogenation reaction using a rhodium catalyst from dehydroamino acid to amino acid has already been well known in general (Chem. Rev., 103, 3029-3070 (2003)). However, many of the reactions are an asymmetric reduction reaction to dehydroamino acid having an alkyl group and an aryl group in a side chain, and there are few examples of a reaction using dehydroamino acid having a substituent with high polarity in a side chain.